A major hazard in water-cooled nuclear reactors is the accumulation of radioactive substances in the structural portions of the reactor. The buildup of radioactive nuclides occurs on the inner surfaces of components which are in contact with the reactor water. This includes both the primary recirculation circuit and the reactor water cleanup system.
During reactor shutdown, workers are exposed to radiation emanating from stainless steel internal walls and inner surfaces of piping. Radioactive materials retained in oxide films which have accumulated on wall and piping surfaces are a major source of radiation exposure. The radioactivity has been found to be predominantly due to the Co-60 isotope. As a result, a substantial effort has been made to identify the key parameters which affect Co-60 buildup and to determine and implement methods for limiting that buildup.
The radiation buildup, controlled mainly by the Co-60 isotope concentration, occurs by two processes. First, the Co-60 which is dissolved in the reactor water incorporates into the crystalline structure of the oxide film as the latter is formed on the stainless steel surfaces. Second, the Co-60 isotope sorbs onto the surfaces of particulates floating in the reactor water or on the fuel. Particles which contain sorbed Co-60 tend to deposit in regions of relatively low water flow velocity. This leads to regions of higher radioactivity which are commonly referred to as "hot spots".
The use of very dilute (trace) concentrations of zinc oxide in the reactor water has been demonstrated, both in the laboratory and in boiling water reactors, to limit the incorporation of Co-60 into the oxide film. Zinc oxide is also used in the cooling water of some nuclear reactors to inhibit intergranular stress corrosion cracking of pipes and internal reactor parts. In some nuclear reactors a slurry of zinc oxide is injected into the reactor feedwater. In other plants a passive system is used where a portion of the reactor feedwater flows through a tank containing zinc oxide pellets. Either process requires a zinc oxide powder of small particle size that is either made into a slurry with water or pressed and sintered into pellets.
Naturally occurring zinc contains Zn-64 isotope which is converted to radioactive Zn-65 in a nuclear reactor. To prevent the formation of Zn-65 isotope, the Zn-64 can be removed from diethylzinc or dimethylzinc by gas centrifuges. The diethylzinc or dimethylzinc is then reacted with water and heated to produce zinc oxide.
Fumed zinc oxide powder made by the metal vapor process can be purchased to meet the requirement for small-sized particles (i.e., .about.0.1 .mu.m) for nuclear power plants that use unisotopically altered zinc in either the slurry or pellet form. Such known powder has a surface area of about 10 m.sup.2 /gm. For plants that use zinc oxide isotopically depleted in the Zn-64 isotope, however, the conventional liquid phase process for converting diethylzinc to depleted zinc oxide produces a powder of much larger average size that requires further physical processing to mill and separate a fine particle fraction suitable for slurry injection or pellet fabrication.